Gantry-type cranes are often outfitted to serve as container cranes for loading standard freight containers into container ships, and also for unloading the containers from the ships. Typically, a container ship has a large number of cells or compartments in which standard freight containers can be received with only a minimum of clearance, and can be stacked vertically until the cells are full. In order to lower a freight container into a cell, the container must be positioned with a high degree of accuracy over the cell so that the container can be lowered directly into the cell without bumping the deck of the ship or the walls of the cell to any objectionable extent. A gantry-type container crane comprises a substantially horizontal supporting structure or boom with rails thereon along which a trolley is moveable in either direction by an electrically controlled power drive. A hoisting means or system is suspended from the trolley and is moveable horizontally therewith. The hoisting system comprises a system of wire ropes hanging downwardly from the trolley and connected to a load carrying device, preferably a spreader bar grasping device for selectively grasping and releasing a freight container.
A container crane is well adapted for unloading containers from railroad cars or semi-trailer trucks and for loading the containers into the cells of a container ship. In a typical sequence of operations, the trolley is moved horizontally along the boom and is stopped directly over a container on a waiting semi-trailer truck. The spreader bar is lowered by the hoisting system into engagement with the container and is actuated so as to grasp the container, which is then hoisted to a safe elevation so that the container will clear any obstacles on the dock or the container ship. The trolley is then moved outwardly along the boom and over the container ship until the trolley is over the cell into which the container is to be loaded. The object of this maneuvering is to enable the container to be lowered by the hoisting system directly into the cell.
However, considerable difficulty has been experienced by crane operators in aligning the container with the cell with sufficient accuracy to enable the container to be lowered into the cell without any objectionable bumping of the container against the deck of the ship or the walls of the cell. This difficulty arises from the fact that the container starts to swing like the bob of a pendulum when the trolley is stopped. The container may swing through several pendulum cycles before the swinging movement is dissipated sufficiently to enable the crane operator to lower the container into the cell. The trolley constitutes the pivotal support for the pendulum. The suspension means are formed by the hoisting rope system, and the bob is formed by the container and the spreader bar. The problem arising from the pendulum swinging of the container has been widely recognized, but no satisfactory solution has heretofore been devised.
U.S. Pat. Nos. 5,713,477 and 5,909,817 granted to Walter J. Wallace, Jr. and Mark A. Wallace, which are hereby incorporated by reference, describe a method and apparatus for controlling and operating a container crane or other similar cranes. The patents disclose a crane having a horizontal boom or other support structure having at least one rail thereon, a trolley along the rail, a rope hoist, and a load carrying device. A control system causes the driven trolley to be stopped momentarily at a first position. The control system then restarts the trolley and stops it at a second position directly over a transfer position. The control system includes an encoder for determining the total length of the pendulum of the crane and the load. The stopping of the trolley at the first position causes the load to swing forwardly into the first quarter of a pendulum cycle. The control system includes means for adjusting the distance between the first and second positions to correspond with the horizontal component traveled by the load during the first quarter of the pendulum cycle in order to minimize any residual swinging movements when the load is stopped.
While these patents represent a significant step forward in terms of speed and accuracy in operating a container crane or the like by minimizing the residual swinging movements of the load, the control system disclosed has a number of limitations. In particular, the control system has the limitation that it is a predictive, open loop control system where the magnitude of the expected pendulum swing is estimated based on the measured length of the pendulum and the estimated weight of the load, the speed of the trolley, and other parameters. When operating within a narrow range of operating parameters, this open loop control system may be sufficient to avoid excessive swinging of the load.
Furthermore, these systems assume a rigid pendulum model; however, the wire rope, rollers and other portion of the crane do not combine to form a device that acts as a rigid pendulum. Instead, these prior art systems act as more a flexible pendulum.
In order to truly eliminate any residual swinging movements when the load is stopped and to allow the container crane and the control system to operate effectively over a broader range of operating parameters, it would be desirable to measure the actual pendulum motion of the load and to operate the control system with closed loop feedback control. It would be further desirable to accurately measure and cancel the swing of the load in a single step prior to the load swinging back and forth, thereby preventing the load from creating the swinging pattern typically addressed by the prior art systems.